Lithium (Li-ion) batteries are being increasingly used as self-contained power sources, in particular in portable equipment (telephones, computers, camcorders, photographic apparatus, tooling, etc.) where they are progressively replacing nickel, cadmium (Ni—Cd) and metal nickel-hydride (Ni-MH) batteries. For several years, sales of Li-ion batteries have exceeded those of Ni-MH and Ni—Cd batteries. This development is explained by continuous improvements to the performance of lithium batteries, in this way giving them mass and volume energy densities that are clearly greater than those provided by Ni—Cd and Ni-MH technologies. While the first Li-ion accumulator batteries possessed an energy density of approximately 80-90 Wh/kg, energy densities close to 200 Wh/kg are from now on obtained (energy density related to the mass of the complete Li-ion cell) As a comparison, Ni-MH batteries possess a maximum energy density of approximately 100 Wh/kg and Ni—Cd batteries have an energy density of the order of 50 Wh/kg.
New generations of lithium batteries are already in the process of development for even more diversified applications (hybrid or all-electric automobiles, energy storage by photovoltaic cells etc.). In order to meet even greater demands for operating and energy potential (per unit mass and/or volume), research has been carried out to develop new materials for Li-ion battery electrodes that are even more efficient and for electrolytes capable of operating over a wide range of potential.
Active electrode compounds used in commercial Li-ion batteries are, for the positive electrode, lamellar compounds such as LiCoO2, LiNiO2 and mixed compounds Li(Ni, Co, Mn, Al)O2 or compounds with a spinel structure such as LiMn2O4 and its derivatives. The negative electrode is generally carbon (graphite, coke etc) or possibly the spinel oxide Li4Ti5O12 or a metal forming an alloy with lithium (Sn, Si etc). The theoretical and practical specific capacities of the positive electrode compounds referred to are respectively approximately 275 mAh/g and 140 mAh/g for oxides with a lamellar structure (LiCoO2 and LiNiO2) and 148 mAh/g and 120 mAh/g for the spinel LiMn2O4. In all cases, an operating voltage close to 4 volts is obtained, relative to metallic lithium.
In addition, it is known that increasing the capacity and/or raising the redox potential of the material of the positive electrode has a greater impact on the increase in total energy density of the battery than an increase of the same order at the negative electrode.
Also, over the last few years, a number of research projects have been undertaken with the aim of providing new positive electrode materials with high-voltage capacity. Recently, positive electrode materials having high-voltage electrochemical activity, beyond 4.2 V versus Li+/Li, have been developed in order to increase the energy density of lithium batteries and possibly to reduce the number of elements to be used in series in applications requiring a high voltage. Among the new promising compounds, the orthophosphates LiCoPO4 and LiNiPO4, spinel oxides of the LiNi0.5Mn1.5O4 and LiNi0.4Mn1.6O4 type and lamellar oxides of the Li(Mn, Co, Ni) O2, Li2MnO3, Li(Mn, Co, Ni)O2 type and their derivatives may be mentioned as examples of compounds functioning, at least partly, above 4.2 V versus Li+/Li or even above 4.5 V versus Li+/Li. In the case of the compound LiCoPO4 for example, the electrochemical activity, corresponding to the oxidation of Co2+ ions, takes place at approximately 4.8 V versus Li+/Li.
Conventional electrolytes used in Li-ion batteries are mainly compounds of a lithium salt, for example chosen from LiClO4, LiAsF6, LiPF6, LiBF4, LiRFSO3, LiCH3SO3, LiN(RFSO2)2, LiC(RFSO2)2, (RF being chosen from a fluorine atom and a perfluoroalkyl group having between one and eight carbon atoms), lithium trifluoromethanesulfonylimide (LiTFSI), lithium bis(oxalato)borate (LIBOB), lithium bis(perfluoroethylsulfonyl)imide (LiBETI), lithium fluoroalkylphosphate (LiFAP), dissolved in a mixture of organic solvents based on cyclic and acyclic carbonates (for example a mixture of methylene carbonate and dimethyl carbonate). Such an electrolyte possesses good ionic conductivity, often greater than 10−3 S/cm, and makes it possible to obtain suitable electrochemical performance (life, autodischarge etc) within a potential window generally between 0 and 4.2 V versus Li+/Li. Apart from liquid electrolytes, dry polymeric electrolytes and gelled electrolytes also exist. The first of these are based on polyethylene oxide or one of its derivatives and have to be operated at approximately 80° C. (that is above the usual temperatures at which telephones, computers and other applications are used) so as to have sufficient ionic conductivity available. The second of these consists of polymers such as PVDF, PEO, PAN and PVC that have soaked up a liquid electrolyte such as those previously described, and are thus subject, in the best cases, to the same potential limitations.
However, under operating conditions above 4.2 V versus Li+/Li these electrolytes are not stable (partial or total oxidation of the electrolyte) and bring about rapid autodischarge of the battery when the latter is partially or totally discharged, this phenomenon being particularly accentuated above 4.5 V versus Li+/Li. The absence of electrolyte capable of operating at a high voltage is thus a brake on the development of high-voltage lithium batteries, and thus of large energy densities.
A few years ago, it was reported that certain liquid solvents of the sulfone type, for example, were stable at a high voltage (XU et al. Electrochemical and Solid-State Letters, 5(1): A26-A29, 2002). Nevertheless, these solvents have not found a practical application in lithium batteries, certainly on account of their reactivity (degradation) at a low potential (below 1 V versus Li+/Li), and their high viscosity at ambient temperature. Moreover, their high voltage stability remains to be demonstrated under actual conditions of use.
Similarly, a class of electrolytes exists, ionic liquids, which were initially considered as stable over a wide voltage range, in particular at a high voltage. Ionic liquids are molten salts composed of large-size organic cations and organic or inorganic anions with a more modest size. A lithium salt, for example LiBF4, LiBF6 or LiTFSI, has to be added to these liquids for use in Li-ion batteries. From a practical point of view, current ionic liquids remain too viscous at ambient temperature and it is generally necessary to add organic solvents in order to reduce the viscosity of the medium.
Dry polymeric electrolytes and gelled electrolytes also exist. The first of these are based on polyethylene oxide or one of its derivatives and have to be operated at approximately 80° C. (above the normal temperatures at which telephones, computers and other applications are used) so as to have available sufficient ionic conductivity. The second ones consist of polymers such a PVDF, PEO, PAN and PVC that have soaked up a liquid electrolyte such as those previously described, and are thus subject, in the best cases, to the same potential limitations.
More particularly, Zhang, Journal of Power Sources, 162: 1379-1394, 2006, reviews various electrolyte additives having one or more functions for improving the performance of lithium batteries. Among these, one category is briefly presented that enables the positive electrode to be protected (cf. p 1385-1386, paragraph 3 “Cathode protection agent”). With this aim in mind, amine compounds (butylamine), imide compounds (N,N′-dicyclohexylcarbodiimide) and amino-silane compounds (N,N′-diethylaminotrimethylsilane) are on the one hand indicated that are capable of reducing either impurities in the water and acids present in the electrolyte and/or electrode, or the too high dissolution of metal ions coming mainly from the positive electrode or current collector, and on the other hand lithium salts are indicated such as LiBOB as electrode additives capable of forming a protective film at the surface of the cathode. However, this document is silent as regards the stability of these additives and their capacity to reduce the problem of autodischarge at high voltages, above 4.2 V versus Li+/Li.
It has therefore been proved that research carried out these last few years will from now on make it possible to obtain a high potential difference between the two electrodes (a negative electrode material operating at a low voltage and a positive electrode material operating at a high voltage). However, to date, no lithium battery electrolyte exists that is truly stable and capable of operating above approximately 4.2 V versus Li+/Li, and in particular between 4.2 and 4.5 V versus Li+/Li under the usual operating conditions. More precisely, the positive electrode/electrolyte interface is unstable at a high voltage and on account of this brings about a rapid discharge when the battery is stored at a high voltage. Autodischarge is an extremely important parameter since it corresponds to the energy loss of a battery after a more or less long storage period and therefore less energy that is still available.